A birth twin would properly be someone who was born in the same place at the same time and on the same date as us, almost like natural twins. However, many similarities can be found between people born with the same criteria except for the place of birth. Indeed, many similarities can be found between people born within a day of each other.

Even natural twins, although being born on the same date and in the same place, can lead very different lives depending on the time they were each born. For instance, one may be born with Jupiter in the first house and the other with the same planet at the same degree of the same sign but in the 12th house. Jupiter would play out differently for both of them due to the difference in the houses and what they govern.

One fascinating example of birth twins is mentioned briefly in Sepharial's book "A Manual of Occultism" (1910). John Hemming the son of an ironmonger was born at the same time, in the same place (St. Martin in the Fields, London) as King George III on 4th June 1738 at 7:48am. John Hemming's father died and he succeeded to the business at the same time that George II died and George III succeeded to the throne. Both suffered temporary loss of their mental faculty. They married on the same day (8 September 1761) and had the same number of children of the same sex. They also died on the same day, 29 January 1820, at almost the same hour.

There are some databases online where you can search for your birth twin and register yourself. You could also try a method that I did. I typed my date of birth into a search engine, typing the month in first then the day and year. I found many, many people who were born on the same date. However, a word of warning here: most of the results returned were of obituaries!